Belt Alternator Starter (“BAS”) powertrain systems for hybrid vehicles differ from conventional, non-hybrid systems in that the torque to turn the accessory drive belt and related accessories is not always generated from the engine crankshaft through the crankshaft pulley. During engine operation, the engine crankshaft provides torque for rotating a Motor Generator Unit (“MGU”), or electric starter generator unit, generally providing for taut and slack sides of the drive belt on opposite sides of the MGU pulley. During engine starting using the MGU, however, the MGU drives the accessory drive belt and related accessories causing the slack side of the belt to switch to the opposite side of the MGU pulley. Accordingly, belt tensioners are generally required in such systems as are idler pulleys that are used to shorten drive belt spans to minimize the potential for drive belt slack.
During engine driven operation of the MGU and other accessories, the loads placed on the drive belt are determined by the power required to drive the accessories, including the MGU unit. The accessory drive loads are relatively light and, accordingly, require moderate to low belt tensioning to avoid belt slippage. On the other hand, during engine starting using the MGU, the MGU must supply power to rotate the engine (crankshaft, pistons, camshafts, etc.) as well as the accessories. MGU driven engine starts require a significantly higher level of belt tensioning to control motion on the slack side of belt and assure that the belt will not slip. Because of packaging requirements it is common to provide separate tensioner and idler arms and pulleys with different pivot locations to attain the necessary drive belt geometry.